1. Field of the Invention
The present invention broadly relates to ejectors for augmentation of thrust and more particularly, is concerned with an axisymmetric ejector configuration which, by using discrete primary air slot nozzles, achieves high thrust augmentation ratios.
2. Description of the Prior Art
The need to improve the effectiveness of thrust generation for vectored thrust VTOL (Vertical Takeoff and Landing) and STOL (Short Takeoff and Landing) aircraft during the takeoff, transition and landing phases of flight has been a recurring problem in the performance of these aircraft. Enormous thrust requirements during these three flight phases have mandated oversized engines which have detracted from cruise performance, and have severely limited payload capability due to excess engine and ducting weight.
The use of a thrust augmenting ejector, integrated into an effective aircraft system, has been considered as a possible solution to this problem. Specifically, additional thrust required for vertical or very short takeoff and landing of an aircraft is generated by diverting a portion of its jet engine exhaust through the thrust augmenting ejector. For this reason, the development of thrust augmenting ejectors (hereinafter referred to simply as ejectors) has been aggressively pursued since their potential was first suggested by Theodore von Karman in his classic paper on the subject in 1949 (see "Theoretical Remarks on Thrust Augmentation," Contributions to Applied Mechanics, Reissner Anniversary Vblume, J. W. Edwards, Ann Arbor, MI, 1949, pp. 461-468). Both theoretical and experimental development of ejectors have been described by Paul M. Bevilaqua in a series of publications (see "Evaluation of Hypermixing for Thrust Augmenting Ejectors," Journal of Aircraft, Vol. 11, No. 6, June 1974, pp. 348-354; "Analytic Description of Hypermixing and Test of an Improved Nozzle," Journal of Aircraft, Vol. 13, No. 1, January 1976, pp. 43-48; and "Lifting Surface Theory for Thrust-Augmenting Ejectors," AIAA Journal, Vol. 16, No. 5, May 1978, pp. 475-581).
The basic conventional ejector is comprised of a shroud or duct and a primary source of high velocity air or fluid aligned therewith. The duct has a contoured convergent inlet section, a constant area mixing section, and a divergent diffuser or outlet section. The primary source of fluid may be high velocity jets of compressed engine bleed air or engine exhaust which are injected into the duct. The high velocity flow of primary fluid entrains ambient secondary air or fluid by viscous shear forces or pressure interaction within the inlet section of the duct and discharges it downstream at the outlet section of the duct. Such secondary air entrainment achieves higher mass and thus higher momentum in the resultant combined primary and secondary fluid flow, which results in more thrust than would have been attained by the simple expansion of the engine exhaust to ambient conditions. The addition of the ejector allows reduction of the engine size by an amount equivalent to the extra thrust generated, after the weight penalty and ducting pressure losses of the added ejector hardware have been compensated for.
Ejector configurations are usually either rectangular (or two-dimensional) as exemplified by the ones illustrated and described in U.S. Pat. No. 3,525,474 to Hans J. P. von Ohain et al, or circular (or three-dimentional) for example as illustrated and described in U.S. Pat. No. 3,739,984 to Remo Tontini. While it is apparent that rectangular ejectors are readily adaptable for integration into an aircraft wing, certain disadvantages are implicit in the rectangular configuration. First, inlet and outlet losses are generated by the corners in rectangular ejectors. Second, flow separation losses often occur in the vicinity of end walls in the two-dimensional rectangular ejector.
These disadvantages of corners and end walls are not present in ejectors having circular configurations. However, very little development effort has apparently been directed toward determining an optimum circular ejector design. Consequently, in view of the general need to improve thrust generation in V/STOL-type aircraft, a particular need exists to design a circular ejector having improved thrust augmentation performance.